Flash boiling

Lionel · April 11, 2015, 10:50

Hello,

I am trying to carry out the simulation of a flash boiling flow in a pipe. It represents the injection duct of an injector, the working fluid is water. We are not interested in the spray generated by the injection, but only the flow inside the injector itself. We want to investigate if the phase change occurs in the injector, for different conditions ( inection pressure, pressure in the chamber, temperature...).
The geometry is a simple cylinder, with an inlet (defined mass flow, and temperature :70°C) and an outlet. The outlet pressure is defined to be between 50 and 250mbar, therefore under the saturation line. Due to the rapid pressure drop, the liquid is superheated and the phase change occurs after a delay ( non equilibrium).
In order to simulate the non- equilibrium, I see two possibilities:

1) Use the cavitation model of CFX, with the IAPWS properties tables for water ( therefore the saturation pressure is function of the temperature). I got some results using this technique, for an outlet pressure of 150 mbar (superheat of 11°C at the outlet) the liquid is subcooled almost till the outlet BC and then suddenly, the phase change occurs and the pressure drops.
When the outlet pressure is 250mbar (superheat of 3°C at the outlet), the phase change doesn't occurs, and the pressure drop is almost linear along the injector lenght.
Do you think those results are physically corrects?
The saturation temperature displayed in the Post processing seems to be clipped to a value to avoid superheat. Does it comes from my simulation, or is it a feature of CFX?

2) The second option I see is to simulate the flash boiling whith the HRM model (Homogenous Relaxation Model) which is not implemented in CFX (but seems to be in OpenFoam). Did anybody tried to implement it? Is it possible to do it through the source terms ( similarly to the tutorial n°31: Steamjet)?

Thank you very much for your help

Lionel

SPH_CFD · July 1, 2015, 10:32

I would like to ask that, why do you try cavitation model instead of evaporation model? Because of saturation liquid at nozzle inlet, I think that we use evaporation model will be better.

April 11, 2015, 10:50	Flash boiling	#1
Lionel New Member Join Date: Jun 2012 Posts: 5 Rep Power: 13	Hello, I am trying to carry out the simulation of a flash boiling flow in a pipe. It represents the injection duct of an injector, the working fluid is water. We are not interested in the spray generated by the injection, but only the flow inside the injector itself. We want to investigate if the phase change occurs in the injector, for different conditions ( inection pressure, pressure in the chamber, temperature...). The geometry is a simple cylinder, with an inlet (defined mass flow, and temperature :70°C) and an outlet. The outlet pressure is defined to be between 50 and 250mbar, therefore under the saturation line. Due to the rapid pressure drop, the liquid is superheated and the phase change occurs after a delay ( non equilibrium). In order to simulate the non- equilibrium, I see two possibilities: 1) Use the cavitation model of CFX, with the IAPWS properties tables for water ( therefore the saturation pressure is function of the temperature). I got some results using this technique, for an outlet pressure of 150 mbar (superheat of 11°C at the outlet) the liquid is subcooled almost till the outlet BC and then suddenly, the phase change occurs and the pressure drops. When the outlet pressure is 250mbar (superheat of 3°C at the outlet), the phase change doesn't occurs, and the pressure drop is almost linear along the injector lenght. Do you think those results are physically corrects? The saturation temperature displayed in the Post processing seems to be clipped to a value to avoid superheat. Does it comes from my simulation, or is it a feature of CFX? 2) The second option I see is to simulate the flash boiling whith the HRM model (Homogenous Relaxation Model) which is not implemented in CFX (but seems to be in OpenFoam). Did anybody tried to implement it? Is it possible to do it through the source terms ( similarly to the tutorial n°31: Steamjet)? Thank you very much for your help Lionel

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July 1, 2015, 10:32		#2
SPH_CFD Member Quang Le Dang Join Date: Jun 2012 Posts: 61 Rep Power: 13	I would like to ask that, why do you try cavitation model instead of evaporation model? Because of saturation liquid at nozzle inlet, I think that we use evaporation model will be better.